The uplink, which constitutes a communication from the mobile station to the base station of the mobile communications system, involves so-called random access type communication in which signals whereby mobile stations individually start a communication are randomly transmitted, or in which mobile stations randomly transmit signals intermittently by means of units known as packets. Therefore, in a case where signals which are transmitted individually by a plurality of mobile stations are simultaneously received by the base station, there is the possibility that collision between these signals will be generated and that the signals will not be correctly received by the base station.
Therefore, with Wideband Code Division Multiple Access (W-CDMA), which is one of the third generation mobile communications wireless access system, a random access type transmission as indicated by “3GPP RAN, 3G TS25.211V3.5.0, December 2000” is performed as follows. First, before transmitting a packet that includes the information symbol sequence to be transmitted, each mobile station transmits a temporally short signal which is spread by means of a predetermined spreading code which is known as a preamble, and the transmission continues until the preamble is detected by the base station or until the number of transmissions reaches to a predetermined number. Further, the base station performs preamble detection and, when a preamble is detected, transmits a corresponding receipt confirmation signal. Also, each mobile station performs packet transmission only upon receiving a receipt confirmation signal from the base station which corresponds with the transmitted preamble.
On the other hand, Multi-carrier/DS-CDMA, which combines multi-carrier modulation and Direct Sequence Code Division Multiple Access (DS-CDMA), has been proposed and investigated. Multi-carrier/DS-CDMA features the division of the available bandwidth into a plurality of frequency bands and DS-CDMA transmission on these frequency bands. To cite an example of Multi-carrier/DS-CDMA, according to cdma2000, which is one of the third generation mobile communications wireless access system, a system in which a 1.25 MHz bandwidth DS-CDMA signal is transmitted in parallel on three carriers in a down-link has become standardized. Further, “Performance of Orthogonal CDMA Codes for Quasi-Synchronous Communication Systems” (V. DaSilva, E. Sousa: Proc. of ICUPC'93, vol. 2, pp. 995-999, 1993), which is an example of an investigation of Multi-carrier/DS-CDMA transmission, proposes a method for performing semi-synchronous transmission that is applied to communications from a mobile station to the base station of the mobile communications system by utilizing the fact that the symbol length of a multi-carrier transmission is larger.
Further, “On the Performance of Multi-carrier DS CDMA Systems,” (S. Kondo and L. B. Milstein: IEEE Transactions on Communications, vol. 44, no. 2, pp. 238-246, February 1996) describes performing a performance evaluation in an environment in which narrowband interference is present and illustrates the fact that characteristics over the whole band are more favorable than those yielded by Single Carrier/DS-CDMA that employs a single carrier. Also, “Optimum Bandwidth per Sub-carrier of Multicarrier/DS-CDMA for Broadband Packet Wireless Access in the Reverse Link” (Suwa, Atarashi, Abeta, Sawahashi: IEICE Technical Report RCS2001-6, April 2001) illustrates the effectiveness of Multi-carrier/DS-CDMA transmission in which the bandwidth is divided.
However, in Multi-carrier/DS-CDMA, a preamble transmission method using multiple carriers has not been established, and there has been a demand for the efficient transmission of a variety of different kinds of information signals by means of preamble transmission that utilizes multi-carrier characteristics.
The present invention was conceived with a view to resolving the above problem, and has, as an object, the provision of a preamble transmission method, a mobile station, a mobile communications system, a preamble transmission program and a computer data signal which permit preamble transmission suited to Multi-carrier/DS-CDMA and which make it possible to efficiently transmit a variety of different kinds of information signals by means of a random access signal by utilizing multi-carrier characteristics.